For disadvantaged children in substandard schools, the recent success of educational adequacy lawsuits in state courts is a welcome development. But the potential of this legal strategy to advance a national goal of equal educational opportunity is limited by a sobering and largely neglected fact: the most significant component of educational inequality across the nation is not within states but between states. Despite the persistence of this inequality and its disparate impact on poor and minority students, the problem draws little policy attention and has evaded our constitutional radar. This Article argues that the Fourteenth Amendment authorizes and obligates Congress to ensure a meaningful floor of educational opportunity throughout the nation. The argument focuses on the Amendment's opening words, the guarantee of national citizenship. This guarantee does more than designate a legal status. Together with Section 5, it obligates the national government to secure the full membership, effective participation, and equal dignity of all citizens in the national community. Through a novel historical account of major proposals for federal education aid between 1870 and 1890, I show that constitutional interpreters outside of the courts understood the Citizenship Clause to be a font of substantive guarantees that Congress has the power and duty to enforce. This history of legislative constitutionalism provides a robust instantiation of the social citizenship tradition in our constitutional heritage. It also leaves a rich legacy that informs the contemporary unmet duty of Congress to ensure educational adequacy for equal citizenship.